1. Field of the Invention
The present invention relates generally to a method of operating a time division duplexing/virtual frequency division duplexing (TDD/virtual FDD) hierarchical cellular system in the context of a cell and sector deployment technique. In particular, though not exclusively, the present invention relates to a method of an unused resource borrowing mechanism and operation in such a network.
2. Background of the Related Art
Most operators have been allocated two or more (FDD) carriers in the recent Universal Mobile Telecommunications System (UMTS) frequency allocations. In principle, an allocation of one pair of FDD carriers enables the operation of only a single network layer. Two paired carriers can cater for a two-layer structure, such as a macro-cell layer together with a micro-cell or pico-cell layer. In hot-spot areas, highly loaded cells can be given extra capacity by adding another carrier to the cell, which would be more effective than increasing the base station transmission power, as described by Jaana Laiho, Achim Wacker, and Tomas Novosad, ed., Radio Network Planning and Optimisation for UMTS, John Wiley & Sons, Ltd., 2002.
One of the promising possible approaches is the TDD and FDD hierarchical cellular structure, which consists of a FDD macro-cell layer and a TDD micro-cell or pico-cell layer. In such a system, the TDD carrier is allocated for supporting hot-spot areas with highly asymmetric traffic with relatively low mobility. In general, the FDD cell and TDD cell use a different carrier in order to cope with inter-system interference. However, as illustrated in FIG. 1, there still exists the adjacent channel interference (ACI) as a system performance degradation factor, as described by Harald Haas, Steve McLaughlin, and Gordon Povey, “The Effect of Interference Between the TDD and FDD Mode in UMTS at the Boundary of 1920 MHz,” in proceedings of IEEE 6th Intern. Symp. On Spread-Spectrum Tech. & Appli., NJIT, NJ, USA, pp. 486-490, Sep. 6-8, 2000. In their work, it has been shown that the effect of interference from the TDD carrier to FDD uplink is more significant and the trade-off for the optimal BS locations.
Another proposal is the exploitation of underused radio spectrum of a cellular CDMA-FDD system (since one communication direction of an FDD interface will be underused) which is named TDD/FDD underlay system proposed by Harald Haas and Gordon Povey, “Apparatus, Method of and System for Improving Capacity in Communication Network,” International Patent Application Number: PCT/GB99/02223, 25 Jul. 1998 and by Harald Haas and Gordon Povey, “Capacity Analysis of A TDD Underlay Applicable to UMTS,” in Proceeding of PIMR99, pp. 167-171. In this idea, a co-existing TDD interface with an unpaired frequency spectrum utilizes the underused FDD frequency band for additional connections, and the TDD is only considered to be operated in the FDD uplink band due to an anticipated channel asymmetry in favor of the downlink. The authors reported substantial capacity gains without a significant deterioration of the quality of service (QoS) and proposed strategies to cope with the additional interference caused by channel borrowing techniques; that is the TDD base station (BS) should ideally located to at a distance of about half the FDD cell radius.
Irrespective of the strategies adopted TDD/FDD underlay system still suffers adjacent channel interference (ACI) at the boundary of TDD and FDD uplink band and the additional interference caused by FDD uplink borrowing technique, since the TDD base station must simultaneously use both frequency bands in order to borrow underused FDD uplink resource. The interference scenario in such a system is illustrated in FIG. 1 where the TDD mobile station (MS) 18 and TDD BS 15 suffer severe interference from the FDD MS 14 with high transmission power at FDD uplink. Conversely, the TDD BS 15 also interferes with the reception of the FDD BS 11. As illustrated in FIG. 1, the ACI types at the boundary of TDD and FDD carrier band are quite similar to that of TDD/FDD underlay system borrowing underused FDD uplink spectrum. For our investigation in this invention, these two interference types such as ACI and the additional interference are taken into account.